Fan blades not turning (turned off) may be blocking airflow. Also check airflow going around radiator instead of thru it at highway speeds. I just fixed a Corvette with the same problem by sealing the top of the rad to the hood, fender to fender.
I would try this first, do you have the rubber piece that mounts to the hood and lays across the radiator support when the hood is closed? Also check any where that air can go around the radiator and close it off some how.